The support comes after a rough two weeks on the campaign trail for Sanford, and Paul acknowledges as much in his endorsement.

"If you’re anything like me, when you hear GOP establishment types coming out of the woodwork to defeat a fellow Republican, it certainly piques your interest," Paul wrote in a letter of support published on conservative website FitsNews.com. "Now, even the national Republican Party is pulling funding for his race!"

Last week, the National Republican Congressional Committee pulled its support for Sanford in the general election. The committee refused to invest in the race after The Associated Press reported that Sanford's ex-wife had accused him of trespassing on her property.

Sanford's campaign is struggling against the Democratic nominee, Elizabeth Colbert Busch, the sister of comedian Stephen Colbert, in a reliably GOP district.

A Paul endorsement often generations strong fundraising for candidates, something Sanford needs. Colbert Busch reported she had raised nearly $900,000 and had about $250,000 in the bank as of April 17, according to her online campaign fundraising report.

Sanford's fundraising report was not available online. But earlier this year, Sen. John McCain's, R-Ariz., Country First PAC donated $2,500 to him and the National Right To Life Victory Fund also spent money to support his bid.

This is an open seat race to replace Tim Scott, a Republican appointed to the Senate earlier this year.